In an image forming apparatus of an electrophotographic system, a formed toner image is transferred onto a sheet of paper using a transfer means. The transfer means includes various methods such as a charger method, a roller transfer method, and a belt transfer method. Of these methods, in recent years, adoption of the belt transfer method to a color image forming apparatus has been proposed in large numbers in terms of separation properties, and the like.
For example, Japanese Patent Application Publication No. H10-268655 proposes to resolve transfer failure such as white spots by making a part of rollers supporting a secondary transfer belt a conductive roller.
Also Japanese Patent Application Publication No. 2002-156808 proposes to press a secondary transfer belt against an intermediate transfer belt at an entrance of a transfer section, or to wind an intermediate transfer belt around a secondary transfer belt in order to resolve a problem caused by pretransfer occurred at an entrance of a transfer section.
Furthermore, Japanese Patent Application Publication No. 2009-156980 proposes to carry out electric discharge on the inside of a secondary transfer belt in order to prevent transfer nonuniformity.
The inventors of the present invention found that, when a long term durability test has been carried out using a tandem type color image forming apparatus adopting a belt transfer method, image failure and the like occurred and, in the case of significant failure, a situation of breakage of the secondary transfer belt occurred. Even if the proposals described in the above Patent Documents were adopted to solve the above problem, the problems such as the image failure were not resolved.
Therefore, as a result of our close examination, it was determined that the failure was caused by undulation of secondary transfer belt. It was found that the undulation was generated as if the undulation came together at the center of the belt from upstream of the belt travelling direction toward downstream, and the undulation phenomena occurred when transfer electrical current was applied. When the transfer electrical current was turned off, no such undulation was generated, and even if it was generated, the degree of the undulation was slight.
If such undulation is superimposed, the surface of the secondary transfer belt is distorted to result in image failure, and if the distortion is considerable, creep deformation (permanent deformation) is caused, which may lead to belt breakage.
Therefore, the purpose of the present invention is to provide an image forming apparatus in which image failure or belt breakage may not occur by resolving problems caused by undulation phenomena during application of transfer electrical current of a transfer belt.